1. Physics 101 - Astronomy Dr. Brian M. Davies
Office: 532 Currens Hall,
Office Hours are M W F 11-noon and Tues 10-11:30 a.m.
See the webpage for notes & syllabus

2. The required course textbook is “Astronomy”, by A. Fraknoi, D
The required course textbook is “Astronomy”, by A. Fraknoi, D. Morrison, and S.C. Wolff Available free online at openstax.org Suggested readings will be posted on my webpage at and on Western Online

3. Openstax Astronomy Week 1 readings
Lecture 1 – Introduction, and the “sky above” (Celestial Sphere)
Section pp
Section pp
Recommended: read the rest of Ch. 1
Lecture 2 – Earth and Sun: Seasons, Solar v. Sidereal Day
Sections pp

4. The Celestial Sphere
The distance to the stars is not evident to our eyes, and they appear to be at the same distance.
If we think of this distance as a radius, the stars appear to be on a sphere, with us (on the Earth) at the center.

5. The Constellation Orion is actually three dimensional, but appears to us as a group of points on the “celestial sphere” For more images, see this link, which will be given in the class notes for the day, available from my website. (link)

6. The Celestial Sphere appears to rotate around us at night
The Celestial Sphere appears to rotate around us at night. But you know that it is the Earth that is rotating. (link)

7. To observers who think the earth is stationary,
The celestial sphere appears to be rotating.

8. The Northern Sky, in a time exposure, shows the apparent motion of the northern part of the celestial sphere around the Pole star, Polaris.

9. Right Ascension and Declination are used to indicate positions on the celestial sphere. They correspond to latitude and longitude on the surface of the Earth.

12. The celestial sphere is oriented with respect to the earth, with poles and an equator.

13. On the celestial sphere we use Declination like we use Latitude on the Earth.

14. On the celestial sphere we use Right Ascension like we use Longitude on the Earth, but measured in hours, minutes, and seconds.

15. For an observer on the ground,
directions are defined in this figure.

18. The observer on the ground can also use angles
for altitude (Alt), the angle up from the horizon, and
azimuth (Azm), which is a heading (N, E, S, W etc.).

19. For more detail on Right Ascension and Declination, see this.

20. Stars are (almost) fixed on the celestial sphere and the Earth rotates inside this sphere. It only looks like the celestial sphere is rotating from our perspective if we are on the Earth.

21. Question ?
To us, which way does the celestial sphere appear to be rotating?
Hint, think about the motion of the moon and the sun during the day or night.
Now, if the celestial sphere is actually not moving, and the Earth is rotating, which way does the Earth rotate?

22. Apparent rotation of the celestial sphere is due to the rotation of the Earth.
The Earth is rotating around an axis that goes from pole to pole through a center.
Eventually, each day, the Sun sets in the west. If we suppose the Sun is the center of the solar system, it is fixed, so:
Each point on the surface of the Earth is going east all the time (except the poles).

23. View of sunset from 100,000 km above the Earth, as it rotates.
I got the latitude and longitude from a web site:
Then I put them into the Earth-Moon viewer:
Start the view 100,000 km above Macomb, IL at midnight UTC, June 7 (in local time, 7:00 p.m., June 6) and get a picture each hour after that as the Earth rotates. See the result in the following slides:

32. Seen from far above the North Pole, the Earth appears to be rotating counterclockwise (CCW).
Sun
If the Sun is directly above point A, then it
is local noon there, and in 24 hours it will
again be noon at that location on the Earth.

33. The Earth is also in orbit around the Sun, taking 365
The Earth is also in orbit around the Sun, taking days to revolve once around.
This orbital motion is also CCW if viewed from above the
north pole.

34. In 24 hours, which is called the solar day, the Earth must rotate more than 360 degrees!

35. Solar vs. Sidereal Day
The solar day is 24 hours long, by definition, but Earth actually rotates through an angle of o in order to be aligned with the Sun. This is due to the orbital motion of the Earth, which means that the Earth has to rotate an additional 360o/365 or 0.986o per solar day.

36. Solar vs. Sidereal Day
The sidereal day is, by definition, the times it takes the Earth to rotate around and come back into alignment with the stars. This is a rotation of exactly 360o and this takes 3.9 minutes less than 24 hours.
1 sidereal day = solar days.

37. Why is this relevant ?
The difference between the solar day and the sidereal day means that the Sun and the stars appear to be going around the Earth at different rates.
The Sun goes around in 24 hours.
Stars go around in 23 hours, 56.1 min.
So the Sun is not in the same place on the celestial sphere day after day.

38. The Zodiac is the set of constellations that the Sun appears to go through during the course of one year.

39. Compare with the zodiac chart in the previous slide.
Right now, Aug. 24, 2017, at 2:40 p.m. the Sun is in Leo, but we can’t see the stars in daylight.
This can be seen on a sky chart if we set the time to sometime in the day, say 2:40 p.m. using
Note: you have to set the date and time to Year 2017 Month 8 Day 24 Hour 14 Minute 40 and click Update. You can also print black on white to save printer ink if you want to take a printed chart outdoors, having set the time to the evening hour when you plan to go out.
Compare with the zodiac chart in the previous slide.
Around midnight, we can go outdoors and see Pisces high in the sky, opposite the Sun from Earth. (next slide)
You can try different months to see that the celestial sphere is in different orientations during the year.

40. Tonight at 11:55 p.m. the sky should look like this
Set the time for 23:55 and notice that we see Pisces and Aquarius high in the sky.
The sky chart also show us that Cygnus is high in the sky at this time, near the zenith.
Depending on the date, we will see some planets along the ecliptic.
For a quick summary of visible planets, etc., see the article “This Week’s Sky at a Glance” in Sky and Telescope magazine:

41. The Ecliptic is the path of the Sun on the celestial sphere, which is tilted with respect to the celestial equator, due to the tilt of the Earth’s axis with respect to our orbit.

42. The axis of the Earth is not perpendicular to the plane of the orbit of the Earth around the Sun. The Earth is tilted by 23.5o.

43. Seasons are due to the changing orientation of the Earth and Sun, not because the Earth is orbiting closer or further from the Sun.

44. Precession of a top
We can demonstrate a type of motion called “precession” by recalling the motion of a toy top (a wobbling motion).
A bicycle wheel can be used to demonstrate precession.
The Earth precesses because it is not a perfect sphere and because there are some forces on it from the Moon and planets.

45. Precession of the Earth

46. Precession of the Earth takes 26,000 years.

47. The North Celestial Pole moves around a circle on the celestial sphere over long periods of time.

48. Motion of the Moon and Phases of the Moon
The development of the current model of the solar system began with careful measurement of the movement of the Sun and the Moon.
To understand this, let’s review the motion and the phases of the Moon, as we currently understand them.
When we watch the Moon, it’s shape changes from one night to the next:

49. From the
astronomy
picture of
the day
web site
( link )

50. Earth and Moon, separately
From Apollo 17 spacecraft
From Earth

51. Earth and Moon, in one picture from the Galileo space probe as it traveled by the Earth on its way to Jupiter.

53. Lunar Phases

54. To do: Get the OpenStax Astronomy book.
Look at the syllabus and the dates.
Mark your calendars for exam dates.
First exam is exactly 2 weeks away!
Enjoy the weekend.